Set the squash on its side and, with a large knife, cut in half vertically.

Trim a piece off the bottom of each squash half so they will lie flat in the pan. Scrape out the seeds and stringy membranes with a large spoon. Place cavity side up in a large roasting pan.

Mix the butter, mustard, and honey in a small bowl until blended. Fill each squash cavity with 2 tablespoons of the butter mixture. Season with salt and pepper. Bake until the squash is very tender, 1 to 1 1/2 hours.